JP2010239036A - Cascade raman resonator and optical fiber laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cascade Raman resonator capable of suppressing the output of unnecessary optical components in a simple structure, and to provide an optical fiber laser. <P>SOLUTION: The cascade Raman resonator is equipped with: an input-side light-reflecting apparatus having a first to n-th input-side light reflectors, to receive excitation light and to selectively reflect the light in each of wavelengths corresponding to first to n-th sets of Stokes light (n is an integer not smaller than 2) of a Raman scattering to the excitation light; a Raman fiber connected to the input-side light-reflecting apparatus to generate Raman scattering light at least by means of the excitation light; and an output-side light-reflecting apparatus having a first to n-th output-side light reflectors connected to the Raman fiber, to selectively reflect the light in each of wavelengths corresponding to the first to n-th sets of Stokes light. A full width at half maximum is not narrower than 2.4 nm in a reflecting wavelength region, at least in one output-side light reflector except the n-th output-side light reflector of all the first to n-th output-side light reflectors. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a cascade Raman resonator and an optical fiber laser using the same.

  Conventionally, a high-power optical fiber laser using a cascade Raman resonator has been disclosed. For example, the optical fiber laser disclosed in Non-Patent Document 1 includes a pumping optical fiber laser composed of an ytterbium (Yb) ion-doped optical fiber laser (YDFL) and a cascade Raman resonator. In this optical fiber laser, the pumping optical fiber laser outputs pumping laser light having a wavelength of about 1117 nm, and the cascade Raman resonator receives the pumping laser light, and stimulated Raman scattering in the cascade Raman resonator. Due to the phenomenon, light whose wavelength is shifted to the long wavelength side is generated one after another, and finally high-intensity laser light having a wavelength of about 1480 nm is generated and output.

S. G. Grubb, et al., "High-Power 1.48 μm Cascaded Raman Laser in Germanosilicate Fibers," in Optical Amplifiers and Their Applications (1995), paper SaA4.

  By the way, the conventional cascaded Raman resonator generates light of, for example, about 1239 nm, 1310 nm, and 1391 nm therein. The light of these wavelengths is consumed inside the cascade Raman resonator as excitation light for finally generating high-power laser light having a wavelength of about 1480 nm.

  However, as the laser light having a wavelength of about 1480 nm output from the cascade Raman resonator is increased in output, the above-described excitation light is output from the cascade Raman resonator as an unnecessary light component together with the laser light having a wavelength of about 1480 nm. For this reason, since removal means such as an optical filter device is required to remove these unnecessary light components, there is a problem that the number of parts of the cascade Raman resonator increases and the configuration becomes complicated.

  The present invention has been made in view of the above, and an object of the present invention is to provide a cascaded Raman resonator and an optical fiber laser that can suppress the output of unnecessary light components with a simple configuration.

  In order to solve the above-described problems and achieve the object, a cascaded Raman resonator according to the present invention receives excitation light, and first to nth Stokes light of Raman scattering with respect to the excitation light (n is an integer of 2 or more). ) And an input-side light reflecting device having first to n-th input-side light reflectors that selectively reflect light of each wavelength corresponding to), and Raman scattering by at least the excitation light. An output side having a Raman fiber that generates light, and first to n-th output-side light reflectors that are connected to the Raman fiber and selectively reflect light of each wavelength corresponding to the first to n-th Stokes lights A full width at half maximum of the reflection wavelength band of at least one output-side light reflector other than the n-th output-side light reflector among the first to n-th output-side light reflectors is 2.4 nm or more. It is characterized by being.

  In the cascade Raman resonator according to the present invention, the full width at half maximum of the reflection wavelength band of the input-side light reflector having a reflection wavelength corresponding to the at least one output-side light reflector is the at least one in the above invention. It is characterized by being narrower than the full width at half maximum of the reflection wavelength band of one output side optical reflector.

  The optical fiber laser according to the present invention includes a pumping optical fiber laser that outputs the pumping light, and a cascade Raman resonator according to the present invention connected to the pumping optical fiber laser. .

  According to the present invention, it is possible to realize a cascaded Raman resonator and an optical fiber laser that can suppress the output of unnecessary light components with a simple configuration.

FIG. 1 is a schematic diagram of an optical fiber laser according to the first embodiment. FIG. 2 is a schematic diagram of the cascaded Raman resonator shown in FIG. FIG. 3 is a diagram illustrating an example of a transmission spectrum of an optical fiber grating. FIG. 4 is a diagram for explaining the operation of the cascade Raman resonator. FIG. 5 is a diagram illustrating an example of an output light spectrum of the optical fiber laser according to the first embodiment. FIG. 6 is a diagram schematically illustrating an optical spectrum of an unnecessary light component to be output when the full width at half maximum of the third optical fiber grating on the output side is changed. FIG. 7 is a diagram illustrating a relationship between the drive current of the semiconductor laser element and the light intensity of the output unnecessary light component in the optical fiber lasers according to the example and the comparative example.

  Hereinafter, embodiments of a cascaded Raman resonator and an optical fiber laser according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. In the following, representative values are shown for the wavelength of light and the reflected wavelength, and are adjusted as appropriate in the vicinity of the values.

(Embodiment 1)
FIG. 1 is a schematic diagram of an optical fiber laser according to Embodiment 1 of the present invention. The optical fiber laser 100 includes a pumping optical fiber laser FL made of a silica glass optical fiber and a cascade Raman resonator CRR made of a silica glass optical fiber connected to the optical fiber laser FL. .

The optical fiber laser FL is a multimode optical fiber 2 that guides pumping light output from the semiconductor laser elements 1 ₁ to 1 _m and the semiconductor laser elements 1 ₁ to 1 _m, where _m is an integer of 2 or more. _{1 to} 2 _m and pumping light guided by the multimode optical fibers 2 ₁ to 2 _m are coupled and output from the double clad optical fiber 5, TFB (Tapered Fiber Bundle) 3, 4, and each double clad optical fiber 5 Double-clad type optical fiber gratings 6 and 7 connected at connection points C1 and C4, double-clad type rare earth element-doped optical fiber 8 connected to optical fiber gratings 6 and 7 at connection points C2 and C3, and TFB4 And a connected output optical fiber 9.

The wavelength of the excitation light output from the semiconductor laser elements 1 ₁ to 1 _m is near 915 nm. In addition, the optical fiber grating 6 has a reflection center wavelength of 1117 nm, a reflectivity of about 100% in a wavelength band having a width of about 2 nm around the center wavelength and its periphery, and light with a wavelength of 915 nm is almost transmitted. The optical fiber grating 7 has a center wavelength of 1117 nm, a reflectance at the center wavelength of about 10 to 30%, a full width at half maximum of the reflection wavelength band of about 0.1 nm, and light with a wavelength of 915 nm is almost transmitted. To do. Accordingly, the optical fiber gratings 6 and 7 constitute an optical resonator for light having a wavelength of 1117 nm. The rare earth element-doped optical fiber 8 is an amplification optical fiber in which ytterbium (Yb) ions, which are amplification substances, are added to the core portion.

  On the other hand, the cascade Raman resonator CRR includes an input side light reflecting device 10 connected to the output optical fiber 9 at the connection point C5, a Raman fiber 11 connected to the input side light reflecting device 10 at the connection point C6, and the Raman fiber 11; The output side light reflecting device 12 connected at the connection point C7 is provided. The connection points C1 to C7 are all fusion spliced.

  FIG. 2 is a schematic diagram of the cascaded Raman resonator CRR shown in FIG. In order to efficiently generate stimulated Raman scattering, the Raman fiber 11 has a small mode field diameter of about 4.5 μm at a wavelength of 1450 nm to enhance optical nonlinearity. The input-side light reflecting device 10 includes first to fifth optical fiber gratings 10a to 10e as input-side light reflectors that selectively reflect light having different wavelengths. The output side light reflecting device 12 includes first to fifth optical fiber gratings 12a to 12e as output side light reflectors that selectively reflect light having different wavelengths, and an optical fiber grating 12f. Regarding the reflection center wavelength of each optical fiber grating, the first optical fiber gratings 10a and 12a are 1175 nm, the second optical fiber gratings 10b and 12b are 1239 nm, the third optical fiber gratings 10c and 12c are 1310 nm, and the fourth optical fiber grating 10d. 12d is 1391 nm, the fifth optical fiber gratings 10e and 12e are 1480 nm, and the optical fiber grating 12f is 1117 nm.

  Here, each of the first to fifth optical fiber gratings 10a to 10e constituting the input side light reflecting device 10 has a full width at half maximum of the reflection wavelength band of 2 nm, and a reflectance at the reflection center wavelength is 99% or more. . On the other hand, for the output-side light reflecting device 12, the fifth optical fiber grating 12e has a full width at half maximum of the reflection wavelength band of about 0.1 nm and a reflectivity at the reflection center wavelength of about 10%. In addition, the other first to fourth optical fiber gratings 12a to 12d and the optical fiber grating 12f all have a full width at half maximum of the reflection wavelength band of 2.4 nm and a reflectance at the reflection center wavelength of 99% or more. .

  FIG. 3 is a diagram illustrating a transmission spectrum of the third optical fiber grating 10c as an example of a transmission spectrum of the optical fiber grating. The vertical axis represents normalized transmittance. As shown in FIG. 3, the third optical fiber grating 10c has a reflection center wavelength of about 1310 nm and a full width at half maximum W of the reflection wavelength band of 2 nm.

Next, the operation of the optical fiber laser 100 will be described. First, in the optical fiber laser FL, when the semiconductor laser elements 1 ₁ to 1 _m output pumping light having a wavelength near 915 nm, the multimode optical fibers 2 ₁ to 2 _m guide the pumping light, and the TFBs 3 and 4 The guided pumping lights are combined and output to the double clad optical fiber 5. The double clad optical fiber 5 propagates coupled pumping light in multimode. Thereafter, the optical fiber gratings 6 and 7 transmit the pumping light propagated through the double clad optical fiber 5 to reach the rare earth element-doped optical fiber 8.

  The excitation light that has reached the rare earth element-doped optical fiber 8 optically excites Yb ions added to the core of the rare earth element doped optical fiber 8 while propagating in the inner cladding of the rare earth element doped optical fiber 8 in multimode. Fluorescence having a wavelength band including 1117 nm is emitted. The fluorescence is amplified by the stimulated emission action of Yb ions while reciprocating in the optical resonator formed by the optical fiber gratings 6 and 7 in a single mode, and oscillates at an oscillation wavelength of 1117 nm. The optical fiber laser FL outputs pumping laser light from the output optical fiber 9.

  The laser light output from the output optical fiber 9 passes through the connection point C5, and is input from the input side light reflection device 10 to the cascade Raman resonator CRR.

  Next, the operation of the cascade Raman resonator CRR will be described. FIG. 4 is a diagram for explaining the operation of the cascade Raman resonator CRR. In the cascaded Raman resonator CRR, the input side light reflection device 10 receives the excitation laser light L1 having a wavelength of 1117 nm from the optical fiber laser FL and inputs it to the Raman fiber 11. Then, the Raman fiber 11 generates Raman scattered light having a wavelength corresponding to the first Stokes wavelength of Raman scattering (hereinafter referred to as first Stokes light) using the laser light L1 as excitation light, and this is Raman amplified. To do. The amplified first Stokes light is subjected to multiple reflection by the optical resonator formed by the first optical fiber gratings 10a and 12a that selectively reflect the corresponding wavelength, and the intensity thereof is increased. Second Stokes light of 1239 nm is generated. Hereinafter, by the same action, the second optical fiber gratings 10b and 12b, the third optical fiber gratings 10c and 12c, the fourth optical fiber gratings 10d and 12d, and the Raman amplification action of the Raman fiber 11 by the respective optical resonators, Third to fifth Stokes lights having wavelengths of 1310 nm, 1391 nm, and 1480 nm are generated. The fifth Stokes light is subjected to multiple reflection by the optical resonator formed by the fifth optical fiber gratings 10e and 12e, the intensity thereof is increased, and laser oscillation occurs.

  Here, in the output side light reflecting device 12, since the reflectance of the fifth optical fiber grating 12e is low, this light having a wavelength of 1480 nm is output to the outside as the laser light L2 from the optical fiber grating 12f. Since the output-side light reflecting device 12 includes the optical fiber grating 12f having a reflection wavelength of 1117 nm, the laser light L1 having a wavelength of 1117 nm output from the optical fiber laser FL is reflected by the optical fiber grating 12f, and the Raman fiber 11 It is used efficiently internally.

  FIG. 5 is a diagram illustrating an example of an output light spectrum of the optical fiber laser 100 according to the first embodiment. In FIG. 5, the vertical axis represents the relative light intensity based on the peak light intensity. As shown in FIG. 5, the output light spectrum includes a component S1 of the laser light L2 having a wavelength of 1480 nm and an unnecessary light component S2 having a wavelength of 1175 nm, 1239 nm, 1310 nm, and 1391 nm.

  Here, in this cascaded Raman resonator CRR, with respect to the pair of optical fiber gratings constituting the optical resonator for each of the first to fifth Stokes lights, the half value of the reflection wavelength band of the optical fiber grating on the output side The whole width is wide. For example, for the third optical fiber gratings 10c and 12c constituting the optical resonator for the third Stokes light having a wavelength of 1310 nm, the full width at half maximum of the reflection wavelength band is 2 nm for the input third optical fiber grating 10c, and the output side The third optical fiber grating 12c is 2.4 nm. As described above, in this cascaded Raman resonator CRR, since the full width at half maximum of the reflection wavelength band of the optical fiber grating on the output side is widened, the output unnecessary light component can be suppressed.

  FIG. 6 is a diagram schematically illustrating an optical spectrum of an unnecessary light component that is output when the full width at half maximum of the third optical fiber grating 12c on the output side is changed. In FIG. 6, the vertical axis indicates relative light intensity, the line L3 indicates the case where the full width at half maximum is 2 nm, and the line L4 indicates the case where the full width at half maximum is 2.4 nm. Further, the scale of the horizontal axis is greatly enlarged as compared with FIG.

  As shown in FIG. 6, the optical spectrum of the unnecessary light component is formed with a recess reflecting the shape of the reflection spectrum of the optical fiber grating on the output side. Further, when the full width at half maximum is 2 nm, a peak P is formed at the end of the hollow on the short wavelength side, and the light intensity is maximum at this peak P. On the other hand, when the full width at half maximum is 2.4 nm, a peak like this peak P does not occur, and the light intensity decreases.

  That is, the cascade Raman resonator uses multistage Raman amplification, and the amplification band of Raman amplification is as wide as about 20 nm. Therefore, in order to obtain a laser beam output of a desired wavelength with a stable intensity, it is necessary to set the reflection center wavelength of each optical fiber grating constituting the optical resonator with high accuracy, and the reflection wavelength bandwidth is also narrow. Is preferred. However, when the reflection wavelength bandwidth is narrow, a peak shown in FIG. 6 may occur in the output unnecessary light component.

  On the other hand, in this optical fiber laser 100, since the full width at half maximum of the reflection wavelength band of the third optical fiber grating 12c on the output side is increased to 2.4 nm, the generation of peaks in the unnecessary light component is suppressed, The light intensity is suppressed. Also, for the first, second, and fourth optical fiber gratings 12a, 12b, and 12d on the output side other than the fifth optical fiber grating 12e, the full width at half maximum of the reflection wavelength band is widened to 2.4 nm. The light intensity of unnecessary light components of each wavelength is also suppressed.

  Further, since the light intensity of the unnecessary light component to be output is suppressed in this way, the light of each wavelength is effectively consumed inside the Raman fiber 11 as excitation light. As a result, the energy conversion efficiency from the excitation laser beam L1 to the laser beam L2 having a wavelength of 1480 nm is also improved.

  In the cascade Raman resonator, the full width at half maximum of the optical fiber grating constituting the optical resonator is set to 2 nm, for example, for the stability of the wavelength and intensity of the laser light output. Therefore, the full width at half maximum of the reflection wavelength band of the first to fourth optical fiber gratings 12a to 12d on the output side is preferably larger than 2 nm, and more preferably 2.4 nm or more.

  Further, in this optical fiber laser 100, for the first to fifth optical fiber gratings 10a to 10d on the input side, the full width at half maximum of the reflected wavelength band corresponds to the corresponding first to fifth optical fiber gratings 12a on the output side. It is narrower than the full width at half maximum of the reflection wavelength band of ˜12d, and is 2 nm. As a result, although the full width at half maximum of the reflection wavelength band of the first to fifth optical fiber gratings 12a to 12d is widened, the stability of the wavelength and intensity of the laser light output is ensured. The full width at half maximum of the reflection wavelength band of the first to fifth optical fiber gratings 10a to 10d is not limited to 2 nm, and may be 2 nm or less.

(Examples and comparative examples)
Next, as an example of the present invention, an optical fiber laser having the same structure as that of the optical fiber laser 100 according to the first embodiment shown in FIG. 1 was produced. The pumping optical fiber laser was configured to output laser light having a wavelength of 1117 nm and an intensity of 6 W and 97 W, respectively, when a drive current of 1 A and 9 A was applied to the semiconductor laser element. The cascade Raman resonator is configured to output laser light having a wavelength of 1480 nm and an intensity of 2 W and 48 W, respectively, when the laser light having the above-described intensities of 6 W and 97 W is input.

  On the other hand, as a comparative example, in the optical fiber laser according to the example, an optical fiber laser in which the first to fourth optical fiber gratings on the output side of the cascade Raman resonator are replaced with one having a full width at half maximum of the reflection wavelength band of 2 nm. Produced.

  FIG. 7 is a diagram illustrating a relationship between the drive current of the semiconductor laser element and the light intensity of the output unnecessary light component in the optical fiber lasers according to the example and the comparative example. In FIG. 7, the wavelength of the unnecessary light component is 1310 nm, and the light intensity is indicated by a level difference from the laser light having a wavelength of 1480 nm.

  As shown in FIG. 7, in the optical fiber laser according to the example, it is confirmed that the level difference is larger than that of the comparative example at any driving current, and the output unnecessary light component is suppressed. It was done.

  In the above embodiment, the wavelength of the excitation laser beam is 1117 nm, and the fifth Stokes light is output from the cascade Raman resonator as the laser beam output. However, there is no particular limitation. Depending on the wavelength of the laser light, the wavelength of the excitation laser light and the order of the Stokes light can be set as appropriate.

  Further, in the above embodiment, all the reciprocal full widths of the first to fourth optical fiber gratings on the output side are widened. However, if at least one full width at half maximum is widened, the effect of the present invention is achieved. It will be a thing. For example, only the full width at half maximum of the optical fiber grating for the unnecessary light component having the highest light intensity or the most unnecessary wavelength may be widened.

1 ₁ to 1 _m semiconductor laser device 2 ₁ to 2 _m multimode optical fiber 3 and 4 TFB
5 Double-clad optical fiber 6, 7, 12f Optical fiber grating 8 Rare earth element-doped optical fiber 9 Output optical fiber 10 Input-side light reflector 10a, 12a First optical fiber grating 10b, 12b Second optical fiber grating 10c, 12c Third Optical fiber gratings 10d, 12d Fourth optical fiber gratings 10e, 12e Fifth optical fiber grating 11 Raman fiber 12 Output side light reflection device 100 Optical fiber laser C1-C7 Connection point CRR Cascade Raman resonator FL Optical fiber laser L1, L2 Laser Light L3, L4 line P peak S1 component S2 Unnecessary light component W Full width at half maximum

Claims (3)

First to nth input-side light reflectors that receive excitation light and selectively reflect light of each wavelength corresponding to first to nth Stokes light (n is an integer of 2 or more) of Raman scattering with respect to the excitation light. An input side light reflecting device having
A Raman fiber connected to the input-side light reflecting device and generating Raman scattered light by at least the excitation light;
An output-side light reflecting device that is connected to the Raman fiber and has first to n-th output-side light reflectors that selectively reflect light of each wavelength corresponding to the first to n-th Stokes lights;
The full width at half maximum of the reflection wavelength band of at least one output-side light reflector other than the n-th output-side light reflector among the first to n-th output-side light reflectors is 2.4 nm or more. Cascade Raman resonator.   The full width at half maximum of the reflection wavelength band of the input side light reflector having a reflection wavelength corresponding to the at least one output side light reflector is smaller than the full width at half maximum of the reflection wavelength band of the at least one output side light reflector. The cascaded Raman resonator according to claim 1. A pumping optical fiber laser that outputs the pumping light;
The cascade Raman resonator according to claim 1 or 2 connected to the pumping optical fiber laser;
An optical fiber laser comprising:

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